This invention relates generally to large turbomachines and in particular to turning gear arrangements for such machines.
Turning gear systems are used with large turbomachines, such as large steam turbine-generators, to cause slow rotation of the turbine rotor whenever the turbine is being started up, shutdown, or at such other times as it becomes necessary to jog the rotor into a different position. The slow rotation during startup and shutdown is carried out to ensure that the rotor remains straight since there is a tendency, particularly at high temperatures, for the rotor to sag between long bearing spans if left for too long in one position.
One example of a decoupling turning gear arrangement is that disclosed in U.S. Pat. No. 3,919,894 to Keeter et al, wherein the turning gear drives the turbine rotor through a bull gear attached to the rotor shaft. Whenever the turbine rotor is at an elevated speed (e.g., above the turning gear speed), the turning gear is automatically disengaged or decoupled from the rotor bull gear. A low speed signal is used to reengage the turning gear on shutdown as the rotor coasts down to zero speed. This is carried out by actuating a pressurized cylinder which moves a gear carriage and which thereby causes the gears to reengage. However, the turning gear may also be manually engaged. A feature of the Keeter el al arrangement is the inclusion of a small auxiliary electric motor in tandem with the large main motor to eliminate gear train backlash prior to rolling the turbine rotor with the larger motor. The Keeter et al patent, U.S. Pat. No. 3,919,894, is hereby incorporated herein by reference.
While turning gears as described above have generally proven to be highly reliable and effective for their purpose, there is a potential for premature disengagement of the turning gear from the turbine rotor on startup since the angular momentum of the rotor can carry it beyond the speed at which the turning gear is disengaged. If this occurs the rotor must then be allowed to coast back down for reengagement. Thus, there is the possibility of having to repeatedly stop and restart the rotor. Furthermore, as the rotor is brought up to turning gear speed from a dead stop or from essentially zero speed, there is a tendency for the rotor to oscillate about its final equilibrium speed. That is, the rotor speed initially overshoots then swings below the equilibrium speed. This is repeated for a number of cycles before the rotor finally settles down. Although the speed oscillations may last for only a few seconds, those of skill in the art have sought to provide apparatus which will effect a smoother startup while avoiding premature disengagement of the turning gear.
Accordingly, it is among the objects of the present invention to provide a turning gear arrangement which does not prematurely decouple from the rotor bull gear and with which final turning gear speed of the rotor is attained without undue oscillations in the rotor speed.
Other objects and advantages of the invention will appear from the ensuing description of the principles and operation of the invention and from the description of a preferred embodiment thereof.